1. Field of the Invention
The present invention relates to an image forming device, in particular to an image forming device such as a laser printer or the like for forming a picture image composed of an assembly of a large number of picture elements arranged in a matrix.
2. Description of the Prior Art
A laser printer is hereinafter described as an example of an image forming device. Generally, in a laser printer, a laser beam for scanning is modulated to be applied or shut off dependently on image data so as to form an electrostatic latent image on a photoconductor drum. The latent image is caused to be visible through development with toner so that the visible image is transferred onto a printing sheet and then fixed (see U.S. Pat. No. 4,862,288 for example).
Since a laser printer is capable modulating laser beams at high speed, it is possible to perform printing of characters and symbols or recording of graphics of such as a postscript with high speed and high quality (i.e. high density) by using a laser printer. Accordingly, laser printers have been widely utilized as output devices for use in various data processing systems or image forming systems in recent years. With wide use of laser printers, it is required to form a smooth image line of a curved line and oblique line portion in a character or graphic figure with high quality.
In such a laser printer, however, since a picture image is formed of an assembly of a large number of picture elements arranged in a matrix, the picture elements located in a curved line or oblique line portion of a character or graph are arranged in a stepped manner so that the image of the curved line or oblique line can not be formed in smooth curved or straight line.
Therefore, in case the density of dots of the picture elements is low, the edge portion of an image is formed in the shape of saw teeth, resulting in deterioration of the quality and smoothness of an image of a character or figure.
Generally speaking, when an image is seen a distance of 12 inches away from the image, the resolution is one by three hundred inch. Therefore, if an image is formed with a dot density of picture elements higher than that of the above mentioned resolution, the above mentioned stepped image line of a curved and oblique line portion is observed to be smooth with good quality although the smoothness and quality of the image is dependent on the inclined angle to the arrangement of the picture elements in a matrix.
However, if the density of the dots of the picture elements is increased, the amount of the image data increases in proportion to the square of the density. Therefore, it becomes necessary to provide a large capacity memory for storing a large amount of image data. Moreover, it takes more time for processing the image in addition to taking much time compared to other processes with the result in that the image forming speed is furthermore lowered.
Moreover, with the high density of the picture elements, it is required to made the accuracy of the parts high, resulting in an increased cost increment.